Taehwang Son

Revolutionizing the FRET-Based Light Emission in Core-Shell Nanostructures via Comprehensive Activity of Surface Plasmons

We demonstrate the surface-plasmon-induced enhancement of Förster resonance energy transfer (FRET)using a model multilayer core-shell nanostructure consisting of an Au core and surrounding FRET pairs, i.e., CdSe quantum dot donors and S101 dye acceptors. The multilayer configuration was demonstrated to exhibit synergistic effects of surface plasmon energy transfer from the metal to the CdSe and plasmon-enhanced FRET from the quantum dots to the dye. With precise control over the distance between the components in the nanostructure, significant improvement in the emission of CdSe was achieved by combined resonance energy transfer and near-field enhancement by the metal, as well as subsequent improvement in the emission of dye induced by the enhanced emission of CdSe. Consequently, the Förster radius was increased to 7.92 nm and the FRET efficiency was improved to 86.57% in the tailored plasmonic FRET nanostructure compared to the conventional FRET system (22.46%) without plasmonic metals.

Surface plasmon-enhanced nanoscopy of intracellular cytoskeletal actin filaments using random nanodot arrays

The feasibility of super-resolution microscopy has been investigated based on random localization of surface plasmon using blocked random nanodot arrays. The resolution is mainly determined by the size of localized fields in the range of 100-150 nm. The concept was validated by imaging FITC-conjugated phalloidin that binds to cellular actin filaments. The experimental results confirm improved resolution in reconstructed images. Effect of far-field registration on image reconstruction was also analyzed. Correlation between reconstructed images was maintained to be above 81% after registration. Nanodot arrays are synthesized by temperature-annealing without sophisticated lithography and thus can be mass-produced in an extremely large substrate. The results suggest a super-resolution imaging technique that can be accessible and available in large amounts.

Localization-based full-field microscopy: how to attain super-resolved images

In this study, we have investigated localization-based microscopy to achieve full-field super-resolution. For localized sampling, we have considered combs consisting of unit pulses and near-fields localized by surface nanoapertures. Achievable images after reconstruction were assessed in terms of peak signal-to-noise ratio (PSNR). It was found that spatial switching of individual pulses may be needed to break the diffraction limit. Among the parameters, the resolution was largely determined by sampling period while the effect of width of a sampling pulse on PSNR was relatively limited. For the range of sampling parameters that we considered, the highest resolution achievable is estimated to be 70 nm, which can further be enhanced by optimizing the localization parameters.

Molecular overlap with optical near-fields based on plasmonic nanolithography for ultrasensitive label-free detection by light-matter colocalization

In this work, we investigate the detection sensitivity of surface plasmon resonance (SPR) biosensors by engineering spatial distribution of electromagnetic near-fields for colocalization with molecular distribution. The light-matter colocalization was based on plasmonic nanolithography, the concept of which was confirmed by detecting streptavidin biotin interactions on triangular nanoaperture arrays after the structure of the aperture arrays was optimized for colocalization efficiency. The colocalization was shown to amplify optical signature significantly and thereby to achieve detection on the order of 100 streptavidin molecules with a binding capacity below 1fg/mm2, an enhancement by more than three orders of magnitude over conventional SPR detection.

Theoretical approach to surface plasmon scattering microscopy for single nanoparticle detection in near infrared region

We present a theoretical approach to single nanoparticle detection using surface plasmon scattering microscopy. Through rigorous coupled wave analysis assuming light incidence on a gold coated BK7 glass substrate under total internal reflection condition for a 200-nm polystyrene as targets attached to the gold film, it was found that surface plasmon polariton induced by incident light on the gold thin film is perturbed. As a result, parabolic waves were observed in the reflection plane. By varying angles of incidence and wavelengths, optimum incident conditions for surface plasmon scattering microscopy were obtained.

Surface plasmon microscopy by spatial light switching for label-free imaging with enhanced resolution

In this Letter, we describe spatially switched surface plasmon microscopy (ssSPM) based on two-channel momentum sampling. The performance evaluated with periodic nanowires in comparison with conventional SPM and bright-field microscopy shows that the resolution of ssSPM is enhanced by almost 15 times over conventional SPM. ssSPM provides an extremely simple way to attain diffraction limit in SPM and to go beyond for super-resolution in label-free microscopy techniques.

Surface-plasmon enhanced microscopy using blocked silver nanodot arrays

Surface-plasmon enhanced microscopy has been investigated using blocked random nanodot arrays. J744 cells were imaged on blocked nanodot arrays under total internal reflection. The images were deconvolved based on localized near-field distribution for super-resolution. Experimentally achieved resolution is estimated to be 100-150 nm. Effects of signal-to-noise and registration on the reconstructed images are discussed.

Quantitative image analysis of angle scanning label-free surface plasmon resonance microscopy

In this report, we describe improvement of image resolution in surface plasmon resonance microscopy (SPRM) which suffers from poor quality due to severe surface plasmon (SP) propagation. Our approach takes two-channel momentum sampling by switched light incidence followed by minimum filtering to implement spatially switched SPRM (ssSPRM). The performance evaluated with periodic wires in comparison with conventional SPRM and bright-field microscopy shows that the effect of SP propagation can be circumvented and the effective decay length of SPRM is calculated to increase by only 7% compared to that of bright-field images.

Ultrasensitive colocalization detection based on plasmonic nanolithography with molecular-overlapped optical near-fields

The detection sensitivity of surface plasmon resonance (SPR) biosensors has been improved by employing colocalization of spatial distribution of electromagnetic near-fields and detection molecules. We have used plasmon nanolithography to achieve light-matter colocalization on triangular nanoaperture arrays and optimized array configurations to improve colocalization efficiency. Streptavidin-biotin interactions were measured to validate the concept. It was confirmed that colocalized distributions of target binding and localized near-fields produced larger optical detection sensitivity. The colocalized detection was also shown to come with wider dynamic range than noncolocalized detection. The effective limit-of-detection of colocalized measurements was on the order of 30 pM. The colocalized detection sensitivity was estimated to be below 1 fg/mm2 in a 100-nm deep evanescent area, an enhancement by more than three orders of magnitude over conventional SPR sensor.

Localized surface plasmon enhanced cellular imaging using random metallic structures

We have studied fluorescence cellular imaging with randomly distributed localized near-field induced by silver nano-islands. For the fabrication of nano-islands, a 10-nm silver thin film evaporated on a BK7 glass substrate with an adhesion layer of 2-nm thick chromium. Micrometer sized silver square pattern was defined using e-beam lithography and then the film was annealed at ~ 200°C. Raw images were restored using electric field distribution produced on the surface of random nano-islands. Nano-islands were modeled from SEM images. 488-nm p-polarized light source was set to be incident at 60°. Simulation results show that localized electric fields were created among nano-islands and that their average size was found to be ~135 nm. The feasibility was tested using conventional total internal reflection fluorescence microscopy while the angle of incidence was adjusted to maximize field enhancement. Mouse microphage cells were cultured on nano-islands, and actin filaments were selectively stained with FITC-conjugated phalloidin. Acquired images were deconvolved based on linear imaging theory, in which molecular distribution was sampled by randomly distributed localized near-field and blurred by point spread function of far-field optics. The optimum fluorophore distribution was probabilistically estimated by repetitively matching a raw image. The deconvolved images are estimated to have a resolution in the range of 100-150 nm largely determined by the size of localized near-fields. We also discuss and compare the results with images acquired with periodic nano-aperture arrays in various optical configurations to excite localized plasmonic fields and to produce super-resolved molecular images.